Apple wants to stop relying on manufacturers A and B. So they develop a new display and the requisite manufacturing process. But then they are going to have manufacturer C take over? What?
It’s like their CPUs and TMSC versus using a Qualcomm Snapdragon. Sure, TMSC is manufacturing Apple Silicon, but Apple always has the option to port their design to a competitors node process or even spin up their own fab if needed (highly unlikely but possible).